Large capacity liquid storage tanks ranging in capacity from 100,000 gallons to over 20,000,000 gallons are frequently built from prestressed concrete panels to ensure long-term structural reliability. Cylindrical liquid storage tanks made from concrete panel walls can be prestressed through the use of wrapping the full circumference of the tank with wire or strands in order to ensure that the tank walls are in compression thereby counterbalancing the force of the liquid on the interior of the tank walls.
During the construction of many large liquid storage tanks, after the wire wrapping, also referred to as the prestressing, of the tank, access to the interior of the tank is required. For instance, such access is required to complete the installation of tank elements such as a pump well and associated equipment such as pumps. In order to provide such access, an underground tunnel can be dug. FIG. 1 illustrates such a tank 10 and an associated underground access tunnel 3 provided to allow access from the exterior of the tank to the tank interior 24. The tank 10 includes a primary, inner tank wall 16 made from concrete panels 18. The inner tank is prestressed using cables 20. Reference numeral 28 refers to a cutaway edge so that the tank interior 24 can be seen. In the tank interior 24 can be seen the interior tunnel access 7 and the tank floor 22. A secondary, outer tank wall 12 surrounds the inner tank. The outer tank is also made from concrete panels 13. Reference numeral 30 refers to a cutaway edge so that the exterior of the inner tank wall 16 can be seen. Both the inner tank and the outer tank are situated on a foundation 14 on the ground 1. The exterior tunnel access 5 is shown as a hole in the ground 1.
The tunnel 3 shown in FIG. 1 is not intended to be shown to scale. Providing such tunnels is often very challenging, involving time-consuming and expensive engineering design work, labor and materials. While having bedrock as a foundation for the tank is desirable from a seismic stability point of view, such ground conditions are quite undesirable for tunneling through because of the hardness of the rock and soil. Tunnels carry the potential risk of at least partial collapse during the excavation and building of the tunnel. Excavating tunnels requires expensive equipment and materials to ensure safety and stability. Once the construction of the tank 10 is completed, the tunnel 3 is no longer needed.
It would be desirable to have a way to provide an opening in the primary and secondary containment tanks during the construction of prestressed panel wall storage tanks so that the practice of providing an underground access tunnel could be avoided.